Apparatus, system, and methods for weighing and positioning wafers

ABSTRACT

An apparatus for characterizing a wafer comprising an aligner comprising a chuck for receiving and rotating the wafer, a sensor for detecting the position of the wafer as it is rotated, a first actuator for lowering and raising the wafer vertically, and a second actuator for moving the chuck horizontally; and a weighing scale comprising a weight sensor disposed proximate to the aligner, and a cantilevered arm extending laterally from the weight sensor over the chuck of the aligner, the cantilevered arm having a through hole surrounding the chuck. The chuck is vertically movable relative to the weighing scale from a first position in which the wafer is supported by the chuck to a second position in which the wafer is supported by the cantilevered arm of the weighing scale. A method for characterizing a wafer using the instant apparatus is also disclosed.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 13/436,848, filed on Mar. 31, 2012, which claims priority fromU.S. provisional patent Application No. 61/472,644 filed Apr. 7, 2011,the disclosures of which are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

Apparatus and methods for characterization of wafers and other objects,such as in the manufacturing of semiconductor wafers.

2. Description of Related Art

Many micro- and nano-scale products, such as integrated circuit (IC)chips and microelectromechanical systems (MEMS), are made according towell known processes. The starting material for such products istypically a thin substrate wafer, such as a silicon wafer substrate. Keysteps in manufacturing ICs and MEMS may include film deposition,photolithographic patterning of features in the film, and processing toremove material and create three-dimensional patterned features on thewafer surface. The film deposition, patterning, and processing steps arerepeated with sequences of materials to form the functional features ofan IC or MEMS device. Current IC chips now have features of less than 50nanometers.

The substrate wafers are processed to produce such products in a rangeof sizes, typically from two inches to twelve inches in diameter.Between the various film deposition, patterning, and processing steps,it is common to perform certain characterization steps for qualitycontrol of these steps, and of the overall final product. For example,both before and after a film deposition, the wafer may be imaged andweighed. By comparing the images before and after the film deposition,defects in the film may be identified. The thickness of the film andother physical characteristics of the process steps can be inferred bycomparing the weights of the wafer before and after the film deposition.

When imaging a wafer, in order to obtain “before” and “after” imagesthat can be easily compared with each other, the wafer must be alignedrelative to a fiduciary mark (such as notch or flat) before imagecapture by a camera, or other optical device. Wafers in need ofcharacterization are typically delivered to a characterization stationin a “cassette,” which is a carrying device having multipleclosely-spaced slots in which the wafers are disposed.

Referring to FIG. 1, a characterization station 10 may be comprised of awafer handling robot 12, a wafer aligner 14, and a wafer imager 16, allof which may be disposed upon a base platform 18. The robot 12, aligner14, and wafer imager 16 may be in communication with and controlled by acomputer 20, which may also acquire image data from the wafer imager 16.

To perform the characterization process of imaging a wafer, the wafer 2is removed from a cassette 22 by the robot arm 13, and disposed upon achuck (not shown) of the wafer aligner 14. The aligner 14 aligns thewafer 2 by rotating it past an optical sensor (not shown) that is incommunication with the computer 20. The center of the wafer 2 is locatedby the computer based on data from the optical sensor. The chuck of thealigner 14 is temporarily lowered, placing the wafer 2 onto a set ofstationary support posts, the aligner chuck is moved so that the centerof rotation of the chuck is aligned with the center of the wafer 2, andthen the aligner chuck is raised and re-engaged with the wafer 2. Thusany translational and angular placement error by the robot is correctedprior to imaging the wafer 2. The wafer 2 is then rotated to a desiredposition relative to its notch or flat under the wafer imager 16. Thewafer imager 16 may include a separate wafer ID reader, which reads andrecognizes a number or other code that is formed at a specific locationon the wafer surface. A full image of the wafer is captured, associatedwith the wafer number, and stored in a memory of the computer 20. Theimage may be further processed with software. The wafer 2 is thenremoved from the aligner and returned to its slot in the cassette 22.The characterization process steps of transporting from the cassette 22to the aligner 14, aligning, code reading, imaging, and returning fromthe aligner 14 to the cassette 22 are repeated for the remaining wafersin the cassette 22.

Subsequently, the cassette 22 of wafers 2 may be moved to a secondcharacterization station (not shown), which includes a weighing scale,and another robot. Thus the steps required to obtain a weight of thewafer 2 are transporting the wafer from the cassette 22 to the scale,weighing the wafer 2, and returning the wafer 2 from the scale to thecassette 22. Alternatively, the characterization station 10 may beprovided with a scale (not shown) that is disposed on the base 18, inwhich case, the robot 12 may move the wafer 2 from the aligner 14 to thescale, and then from the scale back to the cassette 22. A seconddedicated robot (not shown) may be provided to perform the transfers toand/or from the scale.

Regardless of whether the wafer weighing operation is performed at aseparate characterization station, or by a scale provided at the station10, there are problems resulting from operating in this manner.Semiconductor manufacturing and other nanoscale processes are performedin clean rooms. A particle-free environment is maintained to thegreatest extent possible in order to minimize defects on the wafers.Generally, the more separate process operations that are performed, themore particulate contaminants will be generated. Additional waferhandling increases the likelihood of defect causing contamination orphysical damage to the wafers, resulting in costly yield loss.Additionally, performing the operations of aligning, imaging, andweighing separately slows manufacturing throughput, and requiresadditional capital equipment and plant floor space, both of which alsoincrease unit product cost.

In addition to the problems of handling the wafers to and from theweighing station 10, there are problems with determining wafercharacteristics based on wafer weight. Accurate determination of processresults such as film thickness based on weight are subject to influencefrom many variables. These variables may be properties of the materials,used or environmental factors, such as temperature and humidity. Presentstate of the art requires that variables be either determined along withthe wafer weight or recorded with the wafer weight data. In addition,some of the material properties may not be known with high enoughreliability to ensure accurate determinations based on the wafer weight.

What is needed is a single characterization station which can performall of the wafer characterization operations of aligning, imaging, andweighing in a simple and rapid manner.

Along with wafer imaging, current wafer process control relies oninstruments to measure physical properties of the deposited films andfeatures. These instruments are designed to reliably characterizeabsolute properties at specific points across the wafer. The results arecompared with expected values and a quality determination is made.However these measurements require additional instruments and are moretime consuming. Furthermore, each physical measurement is dependent upona high degree of support due to strict calibration and parameter controlrequirements. A wafer characterization system, algorithms and methodsare needed to provide simple ways to achieve process control resultswithout complex computations or apparatus.

SUMMARY

In accordance with the present disclosure, the problem of aligning,weighing, and analysis of a wafer or other similar object at a singlecharacterization station is solved by integrating weight measurementcapability with a wafer aligner in a characterization system. A wafertransport system and an image scanner may also be integrated into thesystem.

In accordance with the present disclosure, there are also providedself-referencing algorithms for identifying wafer process problems. Thealgorithms may be used in conjunction with the characterization system.The algorithms for analysis of the wafer weight data provide a method toidentify wafer process defects without the need for knowledge of thewafer process or materials. The essential concept for the wafer weightanalysis algorithm processes the difference in wafer weight from beforethe process step to the difference in weight following the process step.This change in weight is then compared to the change in weight for adifferent wafer in the lot that has been processed with same process.Additionally, the algorithms may make use of the weight change data as a“fingerprint” for the wafer process. If the data does not match theexpectations for the process step, the wafer or lot is flagged forfurther analysis and action(s). Concurrently, the weight change for thewafer is also used to determine the proper tolerance for the waferweight measurements.

In order to function as desired, the wafer aligner relies on the abilityto manipulate the position of the wafer. Furthermore, due to the factthat the wafer chuck needs to rotate and secure the wafer (with vacuumor other friction means), it is difficult to integrate the weigh celldirectly into the chuck. The Applicants have discovered a solution inwhich an offset arm cantilevers a support for the wafer from theweighing cell, above the aligner. More specifically, in accordance withthe invention, there is provided a wafer characterization apparatuscomprising an aligner comprised of a chuck for receiving and rotatingthe wafer, an object sensor for detecting the position of the wafer asit is rotated, means for lowering and raising the wafer vertically, andmeans for moving the chuck horizontally; and a weighing scale comprisinga weight sensor disposed proximate to the aligner, and a cantileveredarm extending laterally from the weight sensor over the chuck of thealigner, the cantilevered arm having a through hole surrounding thechuck. The chuck is vertically movable by a first actuator relative tothe weighing scale from a first position in which the wafer is supportedby the chuck to a second position in which the wafer is supported by thecantilevered arm of the weighing scale. The chuck is horizontallymovable by a second actuator relative to the weighing scale from a firsthorizontal position to a second horizontal position, thereby enablingthe apparatus to align the center of rotation of the wafer with thecenter of rotation of the chuck. At least a portion of the object sensormay be made movable from a first position proximate to the wafer to asecond position retracted from the wafer, thereby providing clearancefor a camera or wafer imager to be deployed proximate to the wafer. Theapparatus may be further comprised of a wafer cassette weighing cell.

Also according to the present disclosure, a method for characterizing awafer is provided comprising placing the wafer on an aligner comprisinga rotary chuck, determining the offset of the center of rotation of thewafer from the center of rotation of the chuck, moving the chuck andwafer vertically with respect to a cantilevered arm of a weighing scaleto place the wafer on the cantilevered arm, measuring the weight thewafer with the weighing scale, moving the chuck horizontally so as toalign the center of rotation of the wafer with the center of rotation ofthe chuck, and moving the chuck and wafer vertically to remove the waferfrom the cantilevered arm.

Alternative configurations of the Applicants' wafer characterizationapparatus are contemplated. In certain embodiments, the apparatus may becomprised of a weighing scale, and an aligner disposed upon the weighingscale. The weighing scale may include a chuck configured to receive androtate the wafer, a vertical actuator operatively connected to thechuck, and a support for receiving the wafer. In such a configuration,the chuck is vertically movable relative to the support from a firstposition in which the wafer is supported by the chuck to a secondposition in which the wafer is supported by the support. The apparatusmay include a horizontal actuator operatively connected to the chuck,wherein the chuck is horizontally movable by the horizontal actuatorrelative to the support from a first horizontal position to a secondhorizontal position.

A related method for characterizing the wafer may include placing thewafer on an aligner disposed upon a weighing scale and comprising arotatable chuck; measuring the weight of the wafer and the aligner withthe weighing scale; determining the offset of the center of rotation ofthe wafer from the center of rotation of the chuck; moving the chuck andwafer vertically with respect to a wafer support to place the wafer onthe support; moving the chuck horizontally so as to align the center ofrotation of the wafer with the center of rotation of the chuck; andmoving the chuck vertically to remove the wafer from the support. Theweight of the wafer may be determined by subtracting the weight of thealigner from the measured weight of the wafer and aligner.

In other embodiments, the apparatus may be comprised of a rotary chuckdisposed upon a weighing scale, and an aligner comprising a verticalactuator operatively connected to a wafer support. The aligner wafersupport is vertically movable relative to the chuck from a firstposition in which the wafer is supported by the chuck to a secondposition in which the wafer is supported by the wafer support. Theapparatus may include a horizontal actuator operatively connected to thechuck, wherein the chuck is horizontally movable by the horizontalactuator relative to the support from a first horizontal position to asecond horizontal position. The rotary chuck may be comprised of arotatable chuck support operatively connected to a rotary motor.

A related method for characterizing the wafer may include placing thewafer on rotary chuck disposed upon a weighing scale; measuring theweight of the wafer and the rotary chuck with the weighing scale;determining the offset of the center of rotation of the wafer from thecenter of rotation of the chuck; moving a wafer support vertically withrespect to the chuck and wafer to place the wafer on the support; movingthe wafer support horizontally so as to align the center of rotation ofthe wafer with the center of rotation of the chuck; and moving the wafersupport vertically to place the wafer on the chuck. The weight of thewafer may be determined by subtracting the weight of the chuck from themeasured weight of the wafer and chuck.

Also according to the present disclosure, an alternative option foradding a weight sensor to a wafer handling system is provided, in whichthe weight sensor is integrated into a wafer handling robot. The robotmay be comprised of a robot arm comprising an end effector for receivingthe object, and a weight sensor located in the robot arm so as toprovide a signal representative of the weight of the object when theobject is lifted by the arm. The weight sensor may be contained in aweigh cell joined to the robot arm, with the end effector being joinedto the weigh cell. In addition, a further option is to integrate theweight sensor as a separate station. Systems with differing robotconfigurations or other reasons related to manufacturability may be moreoptimal with this configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be provided with reference to the followingdrawings, in which like numerals refer to like elements, and in which:

FIG. 1 is a perspective view of a first wafer characterization stationcomprising a wafer aligner;

FIG. 2 is a front perspective view of a wafer characterization apparatusof the present disclosure, including a wafer being characterizedthereby;

FIG. 3 is a front perspective view of the wafer characterizationapparatus of FIG. 2, shown without the wafer;

FIG. 4 is a rear perspective view of the apparatus of FIG. 3;

FIG. 5 is a top view of the apparatus of FIG. 3 taken along line 5-5 ofFIG. 3;

FIG. 6 is a first front view of the apparatus of FIG. 2 taken along line6-6 of FIG. 2;

FIGS. 7A-7E are front views of the apparatus of FIG. 3 depicting asequence of operations for aligning a wafer on the chuck of an aligner;

FIG. 8 is a side cross-sectional view of the apparatus of FIG. 3 takenalong line 8-8 of FIG. 5;

FIG. 9 is a second front view of the apparatus of FIG. 2 taken alongline 6-6 of FIG. 2, but with a sensing arm of the apparatus raised toaccommodate a wafer imager for imaging the wafer;

FIG. 10A is a front perspective view of a first alternative wafercharacterization apparatus comprising a rotatable scanning arm deployedin the wafer sensing position;

FIG. 10B is a front perspective view of the apparatus of FIG. 10A, shownwith the rotatable scanning arm retracted from the wafer sensingposition to accommodate a wafer imager for imaging the wafer;

FIG. 11 is a lower perspective view of a scale apparatus for weighingcassettes of wafers, the apparatus being combinable with the wafercharacterization apparatus of FIGS. 2-10B;

FIG. 12 is first flowchart depicting a method for characterizing a waferor a set of multiple wafers;

FIG. 13 is a set of bar graphs of weight distributions for various waferfabrication process steps;

FIG. 14 is a second flowchart depicting the determination of acceptablewafers based upon the difference between pre-fabrication step weight andpost-fabrication step weight;

FIG. 15A is a perspective view of a first alternative option forintegrating a weighing means into a robot of a wafer handling system;

FIG. 15B is a top view of the wafer handling robot of FIG. 15A;

FIG. 16 is a perspective view of a wafer handling system in which therobot of FIGS. 15A and 15B withdraws individual wafers from a cassette,weighs them, and may further transport them to another destination;

FIG. 17 is a perspective view of an alternative wafer handling system inwhich a robot transports a wafer to a weigh station for the purpose ofdetermining the wafer weight without alignment or imaging;

FIG. 18 A is a perspective view of a second alternative wafercharacterization apparatus comprising a wafer aligner device disposed ona weigh scale;

FIG. 18B is a side elevation view of the apparatus of FIG. 18A, takenalong the line 18B-18B of FIG. 18A;

FIG. 19 A is a perspective view of a third alternative wafercharacterization apparatus comprising a rotation motor and chuck of awafer aligner device disposed on a weigh scale;

FIG. 19B is a side elevation view of the apparatus of FIG. 19A, takenalong the line 19B-19B of FIG. 19A;

FIG. 20 is a third flowchart depicting a first wafer weight binning lotanalysis scheme that may be practiced with the instant wafer weightmeasurement apparatus; and

FIG. 21 is a fourth flowchart depicting a second wafer weight binninglot analysis scheme that may be practiced with the instant wafer weightmeasurement apparatus.

The present invention will be described in connection with a preferredembodiment. However, it is to be understood that there is no intent tolimit the invention to the embodiment described. On the contrary, theintent is to cover all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

DETAILED DESCRIPTION

For a general understanding of the present invention, reference is madeto the drawings. In the drawings, like reference numerals have been usedthroughout to designate identical elements. In the following disclosure,the present invention is described in the context of its use as anapparatus and system for characterizing a wafer. However, it is not tobe construed as being limited only to use in wafer characterization. Theinvention is adaptable to any use in which it is desirable tocharacterize an object by positioning it and weighing it using a singleapparatus, and optionally obtaining an image of the object or performinganother characterization step. Additionally, the description identifiescertain components with the adjectives “top,” “upper,” “bottom,”“lower,” “left,” “right,” “front,” “back,” etc. These adjectives areprovided in the context of use of the apparatus in characterizingwafers, and in the context of the orientation of the drawings.

The instant apparatus for characterizing a wafer will be first describedwith reference to FIGS. 2-6, and 8-11. Subsequently, methods of and asystem for characterizing a wafer will also be described with referenceadditionally to FIGS. 7A-7E, and FIGS. 12-14.

Referring first to FIGS. 2-6, 8, and 9, a wafer characterizationapparatus 25 is shown. The apparatus 25 is comprised of an aligner 40,which aligns the center of rotation of the wafer with the center ofrotation of the chuck, and a weighing scale 60, which measures theweight of the wafer. The aligner 40 and weighing scale 60 are configuredand integrated at a wafer characterization station so that the steps ofaligning and weighing can be performed by the apparatus 25, without theneed to move the wafer 2 to another location. The aligner 40 andweighing scale 60 may be disposed upon a base plate 26, which may bemade integrally with or mounted to a base similar to the base 18 ofFIG. 1. For the sake of simplicity of illustration, such a base or othersupport fixture is not shown in FIGS. 2-11.

The aligner 40 is comprised of a rotatable chuck 42 for receiving thewafer 2 from a human operator, a robot, or another handling device, andfor rotating the wafer as part of the alignment process. The chuck 42 isrotated by a motor 44. The aligner 40 is further comprised of a firstlinear actuator 46 for displacing the chuck 42 and motor 44 vertically,and a second linear actuator 50 for displacing the chuck 42 and motor 44horizontally. In the embodiment depicted in FIGS. 3-6 and 9, the firstlinear actuator 46 is comprised of a first rotary motor 47, with a shaftdriving a lead screw 48, which is engaged with a threaded drive block49. In like manner, the second linear actuator 50 is comprised of asecond rotary motor 51, with a shaft driving a lead screw 52, which isengaged with a threaded drive block 53. Other means for lowering andraising the chuck and wafer vertically, and means for moving the chuckhorizontally are contemplated, such as linear stepper motors, or smallpneumatic or hydraulic cylinders. The operation of these actuators 46and 50 during the wafer alignment process will be described subsequentlyherein.

The weighing scale 60 is comprised of a weighing sensor, such as acalibrated spring, load cell, or another suitable weight measuringsensor, which is contained in a housing 62 that is proximate to thealigner 40 on base plate 26. The weighing scale 60 is further comprisedof a cantilevered arm 64 that is joined to and extends laterally from asupport pillar 66, which in turn is in communication within the weighingsensor (not shown) in housing 62. The cantilevered arm 64 extendslaterally over the chuck 42 of the aligner 44, and has a through hole 68surrounding the chuck 42. The cantilevered arm 64 is provided with asupport around the through hole 68 for receiving the wafer 2 when thechuck 42 is lowered during the wafer alignment process. In theembodiment depicted in FIGS. 3-6 and 9, the support is comprised of atleast three pins 70 that extend upwardly from the cantilevered arm 64.Other support configurations, such as a solid ring of materialsurrounding the hole, are contemplated.

The simultaneous wafer aligning and weighing steps, which are madepossible by the unique configuration of the instant wafercharacterization apparatus 25, will now be described with reference inparticular to FIGS. 7A-7E. (It is noted that for the sake of simplicityof illustration, the linear actuator 83 of the optical probe 80, whichis shown in FIGS. 2-4, 6, and 9, is not shown in FIGS. 7A-7E.)

Referring first to FIGS. 6 and 7A, a wafer 2 has been placed on thechuck 42 of the aligner 40. The chuck 42 may have a vacuum provision(not shown), or a friction coating such as silicone rubber so as topositively engage the wafer in order to rotate it. It is assumed thatthe wafer 2 has not been placed on the chuck 42 such that the center ofrotation of the wafer 2 and the center of rotation of the chuck 42 arealigned. Therefore, the wafer 2 must be aligned on the chuck 42, andadditionally, the wafer 2 must be rotated to a specific position forimaging. These are two functions of the aligner 42.

In FIG. 7A, the wafer 2 is rotated through at least one full rotation,while the optical sensor 80 detects the position of the edge 3 of thewafer 2 as a function of angular position. The optical sensor may becomprised of a light source 81 and a light detector 82, wherein theamount of light transmitted from the source 81 to the detector 82depends upon the position of the wafer edge 3. Other optical andelectrical sensors, such as a sensor based upon reflected light orcapacitance, may be suitable. As the wafer 2 is rotated 360 degrees pastthe edge sensor 80, data is collected that is, in effect, a map of thewafer edge 3 using polar coordinates. This data is acquired by acomputer (not shown), which can compute the polar offset of the wafer 2.Once that computation is made, the wafer 2 is rotated to the angle ofthe maximum offset, wherein the direction of the maximum offset isaligned parallel to the direction of movement of the horizontal linearactuator 50 as indicated by bidirectional arrow 99 of FIG. 5.

With the amount of the maximum offset now known and so aligned withrespect to the horizontal linear actuator 50, the wafer 2 may now bemoved as a first step in centering it on the chuck 42. Referring to FIG.7B, the horizontal linear actuator 50 is operated as indicated by arrow98, thereby moving the motor 44, chuck 42, and wafer 2 laterally asindicated by arrow 97.

Referring now to FIGS. 6 and. 7C, the vertical linear actuator 46 is nowoperated to lower the horizontal linear actuator 46, turntable motor 44and turntable 42 as indicated by arrows 96, as well as to lower thewafer 2 as indicated by arrow 95. During this lowering, the wafer 2comes to rest upon the support pins 70 of the cantilevered arm 64 of theweighing scale 60. The turntable 42 is separated from the wafer 2. Atthis point in the alignment process, the weight of the wafer 2 isentirely borne by the cantilevered arm 64. Accordingly, at this point,the weight of the wafer 2 is measured by the weighing scale 60. Theweight value may be displayed and/or stored in the computer (not shown).Thus the weight of the wafer 2 is measured simultaneously with the waferalignment process, which is enabled by the integration of the weighingscale 60 and aligner 40 in the instant wafer characterization apparatus25.

Referring to FIG. 7D, during the weighing of the wafer 2, or immediatelythereafter, as it sits upon the pins 70 of the cantilevered arm 64, thehorizontal linear actuator 50 is operated as indicated by arrow 94,thereby moving the motor 44, chuck 42, and wafer 2 laterally asindicated by arrow 93. The amount movement of the wafer 2 and chuck 42in FIG. 7B, and the amount of movement of the chuck 42 in FIG. 7D aredetermined by the computer so as to locate the axis of rotation of thechuck 42 at the same location as the axis of rotation of the wafer 2,i.e., the wafer 2 is precisely centered upon the chuck 42.

Referring to FIG. 7E, the vertical linear actuator 46 is now operated toraise the horizontal linear actuator 50, turntable motor 44 as indicatedby arrows 92, as well as to raise the wafer 2 and turntable 42 asindicated by arrow 91. During this raising, the wafer 2 is removed fromthe support pins 70 of the cantilevered arm 64 of the weighing scale 60,and again carried by the turntable 42. The turntable 42 is then rotatedso as to align the flat or notch of the wafer 2 to a specific locationrequired for imaging of the wafer. Additionally, the wafer 2 may berotated a full 360 degrees, with data acquired by the optical sensor 80.This data may be used to confirm that the wafer 2 is properly centeredon the chuck 42 to within a predetermined specification. If thespecification is not met, the alignment procedure of FIGS. 7A-7E may berepeated.

It is to be understood that the alignment procedure may vary from thatshown in FIGS. 7A-7E. For example, after the wafer offset is determinedin FIG. 7A, the wafer 2 may be lowered onto the pins 70 of thecantilevered arm 64, and weighed. The turntable 42 may then be movedbeneath the wafer 2 to where the wafer and turntable axes of rotationare aligned, and the turntable 42 raised. It is also to be understoodthat the aligner 40 may be configured other than shown in FIGS. 2-9. Forexample, the aligner may operate to contact the edge of the wafer 2, andmechanically push the wafer 2 into the centered position on theturntable 42. The operative requirement is that at some point during thealignment process, the turntable 42 and wafer 2 are separated, such thatthe wafer 2 is supported only by the cantilevered arm 64, therebyenabling a measurement of the weight of the wafer 2 by the weighingscale 60.

In the instant wafer characterization apparatus 25, the cantilevered arm64 replaces the fixed wafer support of the wafer aligner 14 of FIG. 1.This improvement enables the weight sensor to be integrated with thealigner directly, without affecting the aligner performance. It is notedthat by using the cantilevered arm 64, some additional forces areapplied to the support pillar 66 and the weighing sensor. Rather than asimple mass centered on the support pillar 66 and weighing sensor, themass is offset by a horizontal distance. In one embodiment of theapparatus 25, this distance is about 75 mm. The net effect is anadditional moment of force on the support pillar 66 and weighing sensor.The weighing scale 60 can be calibrated to take this moment intoconsideration when determining the wafer weight. In alternativeembodiments (not shown) where the weighing scale is not able tocompensate for the moment of the cantilevered arm, mechanical means isused. For example the cantilevered arm 64 can be extended by about anequal length in the opposite direction; or the cantilevered arm can beprovided with a counterweight of a dense material (such as a leadweight) located on the opposite side of the support pillar 66 tosubstantially balance the arm 64 over the support pillar 66. In any ofthese configurations, the weighing scale 60 of the apparatus 25 isprovided with the capability to take these additional forces intoaccount while still providing a high resolution, accurate weightdetermination of the wafer 2.

With the wafer 2 now weighed, properly centered upon the chuck 42, androtated to the desired angular position, the wafer may be imaged.Referring to FIG. 9, the wafer characterization apparatus 25 may beprovided with an optical sensor 80 having an upper light source (and/orsensing head) 81 that is adjustable as indicated by arrow 90, so as tomake room for a wafer imager 88 to be positioned above the wafer. Theupper light source 81 may be operatively connected to a linear actuator83, which may be comprised of a motor 84 and lead screw 85 that isengaged with a threaded drive block 86. Referring also to FIGS. 10A and10B, in an alternative embodiment, the station 25 may be provided withan optical sensor 80 having an upper light source 81 that is rotatablyadjustable, as indicated by arcuate arrow 89. The upper light source 81may be operatively connected to a rotary actuator 75, which may becomprised of a motor 76 and rotary arm 77.

The instant wafer characterization apparatus 25 may further include awafer cassette weighing cell. By providing such a cell to detect theweight of a cassette, the weight of each individual wafer can bedetermined with a high degree of accuracy when it is removed from thecassette. This can provide a backup or alternative means of obtainingthe weights of wafers by the characterization apparatus 25.

Referring to FIG. 11, the cassette weighing cell 30 may be comprised ofa platform 31 for supporting the cassette 22, a kinematic support 32, aweight sensor 33, at least one lift guide 34, and a linear actuator 35joined to the base plate 26. The linear actuator 35 may be comprised ofa motor 36 and lead screw 37 that is engaged with a threaded lift driveblock 38. In use, the lift drive block 38 lowers the cassette 22 ontothe kinematic support 32, thereby protecting the sensitive weight sensor33 and ensuring an accurate weight measurement. The kinematic support 32ensures that the center of gravity for the cassette 22 is centered onthe weight sensor 33.

An alternative option for adding a weight sensor to a wafer handlingsystem is to integrate the weight sensor into the wafer handling robot.This is best understood with reference to FIGS. 15A-16. Referring firstto FIG. 15A, a wafer handling robot 112 comprising an arm assembly 113is provided. The arm assembly 113 may be comprised of a first arm 114pivotably joined to a second arm 115. A weigh cell 120 is joined to apivotable joint 116 at the distal end 117 of the second arm 115. An endeffector 118 for supporting and articulating a wafer 2 is joined to theweigh cell 120. It can be seen that in operation, the weigh cell 120 islocated in close proximity to the wafer 2 and moves with the robot armassembly 113 and end-effector 118. Such an arrangement will likelyprovide the most accurate wafer weight measurements. However otherarrangements of integrating the weigh cell 120 into the arm assembly 113are contemplated.

To ensure accurate weight measurements of wafers or other objectshandled by the robot 112, the stability of the robot 112 during theweight determination is considered. Robot control software may beprovided, which can adjust parameters to disable servo or other robotactuator adjustments during object weighing. The overall wafer handlingsystem is also structured so as to prevent external influences such asair drafts or vibration from affecting the sensitive weightmeasurements.

The robot 112 with integrated weighing capability may be used in a waferhandling system which further includes wafer imaging capability asdescribed previously herein. The robot 112 may withdraw individualwafers one by one and weigh them while also placing them proximate to acamera, scanner, or other imaging means. Alternatively, the robot 112with integrated weighing capability may be provided in a system thatincludes weight measurement only. Referring to FIG. 16, a wafer handlingsystem 170 is provided in which the robot 112 of FIGS. 15A and 15Bremoves a wafer 2 from a cassette 22, weighs the wafer 2, and thenreplaces it in the cassette 22. The robot 112 may be provided withvertical axis capability to index upwardly and downwardly for access toall of the wafers 2 in the cassette 22, or the robot 112 or cassette 22may be disposed on a vertically operable base (not shown) to provideaccess to all of the wafers 2.

In an alternative embodiment (not shown), for a simple wafer weighingsystem, the robot 112 may be comprised of an arm that is comprised of asingle axis linear actuator. This simplified robot system may provide amore stable platform for the weigh cell 120, thereby improving weighingaccuracy and throughput. The system may further include a second weighcell (not shown) integrated into the wafer cassette pedestal forredundant and/or backup measurement capability.

In an alternative configuration shown in FIG. 17, the aligner, the robot12 and the cassette platform of a characterization station 11 may beseparate devices. This separated configuration may or may not combinethe wafer alignment apparatus with a weigh scale 61. In this system, awafer 2 is transferred from the cassette 22 to the weigh station 61 formeasurement. Depending on the exact system configuration, once the wafer2 is weighed, it is either transferred to the next station (not shown)for imaging or back to the wafer cassette 22.

In accordance with the invention, alternative wafer characterizationapparatus are provided, in which a wafer aligner device, or a portionthereof, is disposed upon a weighing scale. FIG. 18A is a perspectiveview of one such alternative wafer characterization apparatus in whichthe wafer aligner device is disposed on the weighing scale; and FIG. 18Bis a side elevation view of the apparatus of FIG. 18A, taken along theline 18B-18B of FIG. 18A. The apparatus 27 is comprised of an aligner 41and weighing scale 61, with functions as described for the apparatus 25of FIGS. 2-6. The aligner 41 and weighing scale 61 may be disposed upona base plate 26, which may be made integrally with or mounted to a basesimilar to the base 18 of FIG. 1.

The aligner 41 is comprised of a rotatable chuck 42 for receiving thewafer 2 from a human operator, a robot, or another handling device, andfor rotating the wafer 2 as part of the alignment process. The chuck 42is rotated by a motor 44. The aligner 41 is further comprised of a firstlinear actuator 46 for displacing the chuck 42 and motor 44 verticallyas indicated by bidirectional arrow 87, and a second linear actuator 50for displacing the chuck 42 and motor 44 horizontally as indicated bybidirectional arrow 79. The first linear actuator 46 may be comprised ofa first rotary motor 47, with a shaft driving a lead screw 48, which isengaged with a threaded drive block 49. In like manner, the secondlinear actuator 50 may be comprised of a second rotary motor 51, with ashaft driving a lead screw 52, which is engaged with a threaded driveblock 53. Other means for lowering and raising the chuck 42 and wafer 2vertically, and means for moving the chuck 42 horizontally may be asrecited previously for the apparatus 25 of FIGS. 2-6.

The weighing scale 61 is comprised of a weighing sensor, such as acalibrated spring, load cell, or another suitable weight measuringsensor, which is contained in a housing 62 that is disposed on baseplate 26. The weighing scale 61 is further comprised of a weight bearingmember 63 that is in communication within the weighing sensor (notshown) in housing 62. A first structural plate 45 of the aligner 41 isdisposed upon the member 63, so that the weight of the aligner 41 (plusany wafer 2 disposed upon it) may be measured.

The aligner 41 is further comprised of a second structural plate 39,upon which is disposed a support 55 comprised of at least three pins 70for receiving the wafer 2 during the alignment and weighing process. Thestructural plate 39 and support 55 are provided with through holes toaccommodate a rotating shaft of motor 44, which drives chuck 42.

The operation of the apparatus 27, in which a wafer is weighed, andaligned for further characterization steps such as imaging, is similarto that described previously for the apparatus 25 of FIGS. 2-6, as shownin FIGS. 7A-7E. Referring again to FIGS. 18 A and 18B, a robot (such asrobot 12 of FIG. 1) places the wafer 2 onto the aligner 41. The chuck 42may be in a raised position (rather than the lowered position shown inFIGS. 18A and 18B), so that the wafer 2 is supported by the chuck 42.The wafer 2 may then be rotated by the chuck 42. A sensor 80 acquireswafer edge data during rotation as described previously herein forapparatus 25 of FIGS. 2-6. The wafer rotation is stopped at point wherethe wafer offset is parallel to the axis of the horizontal drive screw52.

The vertical drive 46 is then operated to lower the wafer 2 and chuck 42so that the wafer 2 is supported on the support pins 70, as shown inFIGS. 18A and 18B. The horizontal drive 50 is then operated to move thechuck 42 so that the center of rotation of the chuck 42 and the centerof the wafer 2 are aligned. The vertical drive 46 is then operated toraise the wafer 2 and the chuck 42 so that the wafer 2 is supported onthe chuck 42. The centered wafer 2 may then be rotated to a desiredangular position for another characterization step, such as imaging.

At any time during alignment process, and preferably when no motion isoccurring, the weight of the wafer 2 may be measured, regardless ofwhether the wafer 2 is supported by the chuck 42, or by the pins 70. Thetare weight of the aligner 41 may be programmed into the scale 61, sothat it is automatically subtracted out to provide the wafer weight. Theapparatus may be provided with vertical linear guides 57 to aid in thestability of the aligner 41 on the scale 61. These guides 57 are of highprecision and are essentially frictionless so as to not effect waferweight measurements.

At the conclusion of the wafer weighing, alignment, and any othercharacterization steps, the robot 12 (FIG. 1) removes the aligned wafer2 from the aligner 41 and returns it to a wafer cassette 22 (FIG. 1). Itis to be understood that as described for the operation of apparatus 25of FIGS. 2-7E, the order of horizontal and vertical drive operatingsteps may be varied to achieve the same result as described above.

FIG. 19 A is a perspective view of another alternative wafercharacterization apparatus in which a rotation motor and chuck of awafer aligner device is disposed on a weigh scale; FIG. 19B is a sideelevation view of the apparatus of FIG. 19A, taken along the line19B-19B of FIG. 19A. The apparatus 29 is comprised of an aligner 43 andweighing scale 61, with functions as described for the apparatus 25 ofFIGS. 2-6 and the apparatus 27 of FIGS. 18A and 18B. The aligner 43 andweighing scale 61 may be disposed upon a base plate 26, which may bemade integrally with or mounted to a base similar to the base 18 ofFIG. 1. The apparatus 29 of FIGS. 19A and 19B differs from the apparatus27 of FIGS. 18A and 18B in that the aligner 43 of apparatus 29 issupported by the base plate 26, rather than by the weigh scale 61.However, the motor (not visible in FIGS. 19A and 19B, but similar tomotor 44 of apparatus 27), which drives chuck 42, is disposed upon theweight bearing member 63 of the scale 61. Thus the chuck 42 and itsdrive motor are immobile. Instead, to align a wafer 2, the supportmember 57 including at least three support pins 70 is operativelyconnected to the aligner 43, so that a wafer 2 disposed upon pins 70 maybe aligned with the chuck 42.

The aligner 43 of apparatus 29 is similar to the aligners 40 and 41 ofapparatus 25 and 27, and is comprised of a first linear actuator 46 fordisplacing the support member 57 vertically as indicated bybidirectional arrow 87, and a second linear actuator 50 for displacingthe support member 57 horizontally as indicated by bidirectional arrow79. The first linear actuator 46 may be comprised of a first rotarymotor 47, with a shaft driving a lead screw 48, which is engaged with athreaded drive block 49. In like manner, the second linear actuator 50may be comprised of a second rotary motor 51, with a shaft driving alead screw 52, which is engaged with a threaded drive block 53. As shownin FIG. 19A, the threaded drive block 53 and support 57 may be made as asingle piece.

The weighing scale 61 is essentially as recited in apparatus 27 of FIGS.18A and 18B, comprising a housing 62 containing a weighing sensor (notshown), and a weight bearing member 63 that is in communication withinthe weighing sensor.

The operation of the apparatus 29, in which a wafer is weighed, andaligned for further characterization steps such as imaging, is similarto the operation described previously for the apparatus 27 of FIGS. 18Aand 18B. Referring again to FIGS. 19A and 19B, a robot (such as robot 12of FIG. 1) places the wafer 2 onto the chuck 42, when the pins 70 andsupport member 57 are in the lowered position (as opposed to the raisedposition shown in FIGS. 19A and 19B). The wafer 2 may then be rotated bythe chuck 42. A sensor 80 acquires wafer edge data during rotation asdescribed previously herein for apparatus 25 of FIGS. 2-6. The waferrotation is stopped at the point where the maximum wafer offset isparallel to the axis of the horizontal drive screw 52.

The vertical drive 46 is then operated to raise the support member 57and pins 70 so that the wafer 2 is supported on the support pins 70, asshown in FIGS. 19A and 19B. The horizontal drive 50 is then operated tomove the support member 57 and pins 70, thereby correcting the offsetand centering the wafer 2 above the chuck 42 so that the center ofrotation of the chuck 42 and the center of the wafer 2 are aligned. Thevertical drive 46 is then operated to lower the wafer 2 and the chuck 42so that the wafer 2 is supported on the chuck 42, and the pins 70 are nolonger supporting the wafer. The centered wafer 2 may then be rotated toa desired angular position for another characterization step, such aswafer imaging.

At any time during these steps when the wafer 2 is supported by therotatable chuck support 42, and preferably when no motion is occurring,the wafer weight may be measured. The tare weight of the chuck supportand rotation motor assembly may be programmed into the weighing scale sothat it is automatically subtracted out.

At the conclusion of the wafer weighing, alignment, and any othercharacterization steps, the robot 12 (FIG. 1) removes the aligned wafer2 from the aligner 43 and scale 61 and returns it to a wafer cassette 22(FIG. 1). It is to be understood that as described previously, the orderof horizontal and vertical drive operating steps may be varied toachieve the same result as described above.

Other functional stations may be included in the Applicants'characterization apparatus. For example, the apparatus may include awafer tilt wobble station (not shown), which includes a chuck thatgimbals a wafer disposed thereupon using a joystick or other controldevice. Lighting may be directed onto the wafer from a source, in amanner that provides high contrast with any defects that are present. Inone embodiment (not shown), the wobble station may be supported by aweigh scale. In operation, a wafer is placed on the chuck of the wobblestation, and a weight measurement of the station and wafer is made. Theweight of the station is known, and may be subtracted from any weightmeasurement to obtain the weight of the wafer; or the weigh scale may beprovided with the tare weight of the station entered into the scale sothe net wafer weight is measured.

Alternatively, a wafer cassette may be disposed on a weigh scale in amanner similar to the configuration shown in FIG. 11 and describedpreviously herein. With the weight of the entire wafer cassette known,individual wafer weights may be obtained as wafers are added or removedfrom the cassette.

The wafer characterization apparatus 25 of FIGS. 2-11 may be used in anoverall system for characterization and analysis of lots containingmultiple wafers. The overall system further includes a computer insignal communication with the aligner 40, weighing scale 60, and imager88 of the apparatus 25. The computer outputs signals for control of thealigner 40, weighing scale 60, and imager 88, and acquires datatransmitted back from these devices. The computer stores the data in amemory. The computer is further comprised of a processor, which canexecute algorithms stored in memory for analyzing the data, and formaintaining a database also stored in memory.

FIG. 12 is a flowchart depicting a method 200 for characterizing a waferor a set of multiple wafers using the Applicants' apparatus 25 andoverall system. The Applicant's system and method 200 differs from priorprocesses by using a “less is more” approach to wafer measurements. Thesystem and method 200 are designed to provide a simple pass/failmeasurement with respect to a particular measured parameter. In order toachieve the pass/fail measurement, the system and method 200 avoidsusing weight measurements for absolute determinations. By avoiding thecomplexities of absolute determinations, the requirement for referencestandards and critical parameter controls are effectively canceled out.Once the pass/fail measurement is made in accordance with the method200, the determination and interpretation of the measurement data areleft to the engineer/operator of the system. In practice, this techniqueis very powerful for manufacturing process control.

In addition to the in-process wafer analysis, as the wafers areprocessed, the Applicants' system creates a database with images andresults for each individual wafer. The database is a permanent recordthat enables process engineers to communicate, review, and examine eachwafer via a computer communications network. When the system is used tocollect the image data before and after each process step it becomes avery powerful means to review and identify wafer process integrity. Thecareful recording of all the data enables the Applicants' system tomeasure and monitor each process step with identical analysis.

The potential power of this system becomes apparent when defect/failureanalysis requires historical review of the wafer and end productprocess. For example, consider a situation where semiconductor chips aremade from finished wafers, and where, after fabrication, an individualchip is identified as defective. Using the database generated by theApplicants' system, an engineer can find that exact location on a wafer,and then visualize and replay each step of the process from his desk. Ashe looks back through the process history, he can identify the source ofthe defect—possibly a particle, a scratch, or any number of otherproblems.

The same principle applies to defects for batches or lots of products.For example; if a lot of parts have suddenly tested near or out oftolerance, the Applicants' system can be used to examine the wafer orwafers for wafer-level symptoms. Once a particular process step isidentified as causing the problem, the information can be fed back intothe process to correct the problem.

A basic premise of the Applicants' system and method 200 is to provide asimple means to achieve a process measurement result without complexcomputation or setup. Alternative systems that strive to provideabsolute determinations are prone to tolerance variances and/orcalibration issues, with many more dependent variables. In contrast, theApplicants' system and method 200 are self-calibrating andself-referencing.

By way of illustration, and not limitation, consider that the effect onwafer weight by the different fabrication processes is a complicatedmatrix of process type, film parameters, and other variations. In orderto make a useful absolute measurement of the film or process step,precise parameters such as the density of the film material would berequired. As a consequence, the integration of wafer weightdetermination as a means for control of absolute characteristics becomesdifficult.

The Applicants' method 200 utilizes differential comparison to providehigh resolution determinations with a high degree of confidence, withoutthe need for strict parameter control. Referring to FIG. 12, the method200 is applicable to any fabrication process step which affects thewafers. In steps 205-230, the first wafer in each lot of wafers ischaracterized each time the wafer is inspected. The characterization mayinclude the concurrent aligning and centering 210 of the wafer on thealigner 40, and determining 215 the weight of the wafer by the weighingscales 60; reading 220 the wafer identification number or code using asensor and optical character recognition (OCR) or barcode or other codereading software; and acquiring 230 an image of the wafer, as describedpreviously herein. These steps may be done in a different order. Forexample, it is not necessary that the aligning and centering 210 of thewafer be done in parallel with determining 215 the weight of the wafer.The aligning and centering 210 may be done, followed by reading 220 thewafer ID number, followed by determining 217 the weight of the wafer(shown in dotted line).

For the first wafer of the lot, the weight change (i.e., the differencebetween the post-process step wafer weight and the pre-process stepwafer weight) is recorded, and this value is defined 240 as a referencevalue. Each subsequent wafer in the lot has its weight measured beforeand after the wafer processing step as well. The post/pre-processdifference in weight of each wafer is compared 260 to thepost/pre-process difference in weight for the reference wafer. Thedifferences are identified 270 and a simple tolerance computationdetermines if the wafer should be flagged for re-examination. The waferimage may also be compared 280 to the reference image.

Additionally, for the first wafer of the lot, the post-process stepwafer image acquired in step 230 is defined 250 as a reference or“golden wafer” image. The image for each subsequent wafer in the lot iscompared to this image. This part of the process does not compute thedifference between pre- or post-wafer processing.

The method 200 uses a unique algorithm that gleans important informationfrom the wafer weight without the need for complex parameters sets thatdescribe the wafer, film, and process step. Since each wafer ischaracterized before and after each process step, the Applicants' systemcan record the relative increase or decrease for each individual wafer.Furthermore, the algorithm incorporates the weight change of thestandard “golden wafer” as well, as follows:ProcessWeight=weight of wafer pre-process−weight of wafer post processandWeightDetermination=ProcessWeight_(first wafer)−ProcessWeight_(current wafer)

Once the result is determined, the process software will flag the waferfor reexamination based on pre-determined tolerances. The uniquecomponent of the weight analysis is the ability of the Applicants'system to compare changes to the target wafer with changes to thereference wafer. This yields meaningful results for any process stepwithout the need for any information about the actual film or process.This principle is illustrated generally in FIG. 14.

TABLE 1 Hypothetical sample wafer data to which the algorithm may beapplied. Wafer Weight (grams) Wafer Before Change in Num Process PostProcess Weight Change from Reference 1 50.079 55.010 4.931 “GoldenWafer” 2 50.084 55.022 4.937 0.006 3 50.075 55.048 4.973 0.041 4 50.09455.161 5.067 0.136 5 50.046 55.019 4.973 0.042 6 50.024 55.026 5.0020.071

The method 200 may also be illustrated with reference to TABLE 1, whichcontains a hypothetical set of wafer data. In the sample data of TABLE1, there are 6 wafers. Wafer 1 has been arbitrarily selected as the“golden wafer”. The “golden wafer” reference could have just as easilyhave been one of the other wafers. TABLE 1 shows wafer weight databefore and after a particular process step. The change in weight iscomputed and shown. The last column shows the change in weight relativeto the reference or “Golden Wafer,” i.e., wafer 1. From the data one cansee that depending upon the allowable tolerance some of the wafers canbe flagged for review, rejection or rework. If a tolerance of 0.050 wereset, then wafer 4 and 6 would be flagged.

In practice, applying the above formulae with general parameters to thewafer weight in order to arrive at a conclusion will not always providea satisfactory result. The difficulty is that the weight tolerance isdifferent for varying wafer fabrication process steps. Thicker filmscoated on the wafer will be heavier, and thinner layers will be lighter.Accordingly, the tolerance range varies between these types of layers,and how the weight change relates to the physical change in waferproperties and the process step. In order to address this problem, thetolerance can be expressed in terms of a factor or a percent error. Inpractice, the percentage tolerance will not apply universally either.

The Applicants have discovered that in the process of wafer fabrication,the change in wafer weight is analogous to a “fingerprint” of theprocess. For a given change in wafer weight, a particular process stepcan be identified. The Applicants' system and method 200 applies thischaracteristic to create a table of weight bins, as depicted in FIG. 13for a series of wafer fabrication process steps A-E. This scheme ofranging or binning the wafer weight change in effect provides a means toapply the correct parameters to each measurement without requiringcomplicated recipes for each wafer. As each weight result is determinedusing the pre/post process algorithm previously described, the toleranceparameter is determined by this result. For example, using thehistograms in FIG. 13, comparing the width of the weight bins shows thatthe acceptable tolerance for a weight result that falls in the range forPROCESS B would be less than for a weight result that falls in the rangefor PROCESS E. Accordingly, the Applicants' system and method 200 becomeself-referencing and self-calibrating.

Utilizing a wafer weight binning lot analysis scheme with the instantwafer weight measurement apparatus, a very simple system may be createdwithout the need for wafer alignment or the use of an integrated waferID reader. Such a system may be used in performing analyses of weightmeasurements made before and after various film deposition, patterning,and etching process steps that are commonly performed on wafersubstrates. Advantages of such a system are lower cost, less waferhandling, and higher throughput.

Either of two schemes may be used in the weight analysis. The firstscheme, depicted in FIG. 20, is a wafer by wafer scheme that tracks eachwafer and compares the weight against a reference or “golden wafer.” Thesecond scheme, depicted in FIG. 21, uses the mean weight of the lot as areference.

Referring first to FIG. 20, for a given process step (coating,patterning, etching, etc.), the first wafer scheme 300 starts bymeasuring 310 the first wafer, or “golden wafer,” before 324 and after326 the process step. (It is to be understood that the terms “firstwafer,” “reference wafer,” and “golden wafer” terms used in thisdescription refer to the practice of using data from a particular waferas the basis of comparison of data from other wafers.) The differencebetween the post- and pre-step weights is computed 330 and becomes thereference difference assigned 334 to the first wafer. Each wafer in thelot is subsequently weighed 320 before 324 and after 326 each processstep, and the difference between the post- and pre-step weights iscomputed 330. For any given wafer that follows the first wafer, thedifference between the post- and pre-step weights is compared 336against the reference weight difference 334. If the difference betweenthe post- and pre-step weights is within a pre-determined tolerancerange from above and below the reference difference assigned 334 to thefirst wafer, the wafer under consideration is qualified 340 to continueto the next step in the process. Deviation beyond the pre-determinedtolerance will be used to flag an error condition based on thesecomparison criteria, in which case the wafer will not continue to thenext step in the process. The out-of-spec wafer may be discarded and/orfurther tested, imaged, or inspected to understand the cause of thevariance beyond the tolerance. It is noted that this scheme relies onthe assumption that the first (“golden”) wafer will always be present inthe lot as the lot progresses through the various steps in the process.The reference or “golden” wafer does not necessarily need to be thefirst wafer. If it happens to be missing either due to a split lot,damage, or rework, then a different wafer can be used as the referencesince the raw data collected for each wafer is the same.

Referring now to FIG. 21, the second wafer weight reference scheme 400is comprised of a lot analysis. In the lot analysis, the weight of eachwafer in the lot is measured 410 before 412 and after 414 the processstep. Once the measurements are complete, the difference between themean weight before and the mean weight after the process is computed420. To improve the data consistency, any points that statistically fallout a defined tolerance range may be ignored in the referencecomputation. By correlating the mean weight change data and the databaseof weight bins, the system can determine 430 the wafer process step witha reasonable amount of certainty. Determination of the process step isin itself an additional data point for process control. If the systemidentifies an out of sequence process step, the error is identified andcorrective action can be performed to rework or reject the wafer.Additionally, the determination of the process step allows acceptancecriteria to be used based on that particular process step.

Once the process step is known, the change in weight of each individualwafer is compared against the bin values. This final comparison 440determines a pass/fail code for each wafer that contains relevantinformation about the deviation determined from the process step anderror magnitude. This scheme 400 is much more flexible and robust inthat knowledge of the individual wafer number is not required to producean accurate determination of its acceptability.

The weight data for each wafer and process step is recorded as well asthe reference weight for the qualification. This information may belinked in the database with the wafer imaging and manufacturing data toprovide a complete record of the wafer history throughout themanufacturing of the wafer and products resulting from it.

The basic weight measurement algorithm previously described withreference to FIGS. 12-14 is also applicable to the above description ofwafer weight and pre- and post-process change in weight. The weightvalue is the change in the difference for each wafer. The instant systemcompares the wafer weight before and after processing to arrive at aweight change value. Each wafer that is examined has a weight change.This alone does not represent much value as discussed previously, due tothe possibility of variance in physical and environmental variables.However, when the change in weight is compared with the change in weightof a reference, the process variables cancel out, and the result isreliable data that is a sensitive indicator of wafer process control.

The wafer weight data may be correlated with other wafer physicalproperties or information. As each wafer passes through the Applicants'wafer characterization system, wafer edge data may be acquired alongwith the image and weight information. Based on the wafer weight, thecharacterization system can analyze additional information to helpclassify a given wafer. The weight data alone, as described previously,may not be reliable without strict environmental and physical control.However, using the weight change binning scheme disclosed herein,correlation and classification of wafer physical anomalies can bedetermined.

It is, therefore, apparent that there have been provided, in accordancewith the present invention, apparatus and methods for characterizationof wafers and other objects. Having thus described the basic concept ofthe invention, it will be rather apparent to those skilled in the artthat the foregoing detailed disclosure is intended to be presented byway of example only, and is not limiting. Various alterations,improvements, and modifications will occur and are intended to thoseskilled in the art, though not expressly stated herein. Thesealterations, improvements, and modifications are intended to besuggested hereby, and are within the spirit and scope of the invention.Additionally, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations therefore, is notintended to limit the claimed processes to any order except as may bespecified in the claims.

We claim:
 1. A method for characterizing a wafer comprising: a) placingthe wafer on an aligner comprising a rotary chuck; b) determining theoffset of the center of rotation of the wafer from the center ofrotation of the chuck; c) moving the chuck and wafer vertically withrespect to a through hole of a cantilevered arm of a weighing scale toplace the wafer on the cantilevered arm with the through holesurrounding the chuck; d) measuring the weight the wafer with theweighing scale; e) moving the chuck horizontally so as to align thecenter of rotation of the wafer with the center of rotation of thechuck; and f) moving the chuck and wafer vertically to remove the waferfrom the cantilevered arm.
 2. A method for characterizing a wafer, themethod comprising: a) placing an aligner having a weight upon a weighingscale; b) placing the wafer on a rotatable chuck of the aligner; c)measuring the weight of the wafer and the aligner with the weighingscale; d) operating the rotatable chuck of the aligner to rotate thewafer while measuring wafer position, and determining the offset of thecenter of rotation of the wafer from the center of rotation of thechuck; e) moving the chuck and wafer vertically with respect to a wafersupport to place the wafer on the support; f) moving the chuckhorizontally so as to align the center of rotation of the wafer with thecenter of rotation of the chuck; and g) moving the chuck vertically toremove the wafer from the support.
 3. The method of claim 2, furthercomprising determining the weight of the wafer by subtracting the weightof the aligner from the measured weight of the wafer and aligner.
 4. Amethod for characterizing a wafer, the method comprising: a) placing arotary chuck having a weight upon a weighing scale; b) placing the waferon the rotary chuck; c) measuring the weight of the wafer and the rotarychuck with the weighing scale; d) operating the rotary chuck to rotatethe wafer while measuring wafer position, and determining the offset ofthe center of rotation of the wafer from the center of rotation of thechuck; e) moving a wafer support vertically with respect to the chuckand wafer to place the wafer on the support; f) moving the wafer supporthorizontally so as to align the center of rotation of the wafer with thecenter of rotation of the chuck; and g) moving the wafer supportvertically to place the wafer on the chuck.
 5. The method of claim 4,further comprising determining the weight of the wafer by subtractingthe weight of the chuck from the measured weight of the wafer and chuck.6. The method of claim 1, further comprising using a cassette weighingcell to measure the weight of a wafer cassette containing the wafer anda plurality of additional wafers, removing the wafer from the wafercassette, measuring the weight of the wafer cassette and the pluralityof additional wafers, and determining the weight of the wafer bysubtracting the weight of the wafer cassette and the plurality ofadditional wafers from the weight of the wafer cassette containing thewafer and the plurality of additional wafers.
 7. A method forcharacterizing a wafer comprising: a) measuring the weight of a wafercassette containing a plurality of wafers; b) removing a first waferfrom the wafer cassette and placing the wafer on an aligner comprising arotary chuck; c) determining the offset of the center of rotation of thewafer from the center of rotation of the chuck; d) moving the chuck andwafer vertically with respect to a through hole of a cantilevered arm toplace the wafer on the cantilevered arm with the through holesurrounding the chuck; e) moving the chuck horizontally so as to alignthe center of rotation of the wafer with the center of rotation of thechuck; f) moving the chuck and wafer vertically to remove the wafer fromthe cantilevered arm; g) measuring the weight of the wafer cassette andthe plurality of wafers after removal of the first wafer; and h)determining the weight of the first wafer by subtracting the weight ofthe wafer cassette and the plurality of wafers after removal of thefirst wafer from the weight of the wafer cassette containing theplurality of wafers.